1. Field of the Invention
Embodiments disclosed herein relate generally to methods of treating wells containing hydrogen sulfide.
2. Background Art
Natural gas is a mixture of hydrocarbons and non-hydrocarbon gases found in geologic formations beneath the earth's surface, often in association with petroleum. As obtained from the ground, raw or sour natural gas contains a number of impurities, namely water, carbon dioxide, hydrogen sulfide, organic sulfides, and condensable hydrocarbons, such as propane, butane, and pentane, which must be removed before being introduced into a pipeline. When idle, and particularly when the hydrocarbons are accompanied by high sulfur concentrations, hazardous sulfur compounds will accumulate at the wellbore, often at pressures significantly above atmospheric pressure. When this buildup of hazardous sulfur compounds occurs, the hazardous compounds can be released into the environment, particularly when maintenance of the well for workover or abandonment purposes is required. Furthermore, it is during such maintenance that there exists the greatest risk of injury and death to maintenance workers and the greatest risk of hazardous emissions to the environment.
Generally speaking, hydrogen sulfide and organic sulfides (collectively here “sulfides”), because of their corrosiveness and toxicity, are typically removed from natural gas in the field prior to introduction to a pipeline for transport to a market or off-site processing plant. The maximum level of hydrogen sulfide, “H2S,” the primary sulfide impurity of natural gas, permitted to be introduced into a pipeline is often limited to 4 parts per million (ppm), although in some limited instances, up to 16 ppm is acceptable.
For producing wells, that is wells from which fluids are withdrawn, various methods and processes are, and have been, employed to treat hazardous sulfur compounds to prevent their release to the environment. According to some processes, such treatments are conducted at a time when the sulfur compounds are dissolved or dispersed in or otherwise intimately associated with fluid hydrocarbons after or during the removal of the fluid hydrocarbons from the well and before the fluid hydrocarbon streams are subjected to refinery operations, used as a fuel, or used as a precursor for subsequent operations. Such operations are referred to herein as “upstream” treatments. Other treatments, referred to herein as “down stream” treatments, are conducted after the fluid hydrocarbons have been refined, or prepared for use as a fuel, or employed as a precursor for the manufacture of hydrocarbon-based products.
Hydrogen sulfide, like hydrocarbon components of natural gas, exists in the gaseous state at normal temperatures and pressures. There are a variety of approaches for removing hydrogen sulfide from, or at least substantially reducing the amount of hydrogen sulfide in natural gas, i.e., “sweetening” natural gas. One general approach is to expose the raw natural gas to a treatment liquid containing an agent which chemically reacts with hydrogen sulfide, a so-called a hydrogen sulfide scavenger. Usually, the natural gas is bubbled through the treatment liquid, and as the bubbles rise through the treatment liquid, hydrogen sulfide comes into contact with the agent in the liquid to react and form a non-gaseous, or relatively non-volatile, product. The hydrogen sulfide thus becomes trapped in the liquid phase, and is thereby removed from the gas. In other schemes, a portion of a sulfur-rich gas may be employed to aspirate a treatment fluid and pass the admixed stream through a mixing zone, or an atomizing nozzle before the admixture of gas and treatment fluid is contacted with the fluid in a wellbore, in a pipeline, or in a vessel. Such systems are characterized by operations which employ pumps and compressors to maintain the pressure at the wellbore during the treatment process. When working on a wellbore that is known to have accumulated hazardous sulfur compounds, fluids or water, known as “kill fluids” or “kill water,” with a sweetening agent such as triazine, is typically injected into the wellbore so as to raise the hydrostatic pressure in the wellbore and reduce or arrest further influx of said hazardous sulfur compounds into the wellbore.
There are a number of characteristics that are considered desirable in treatments containing such active agents. Generally speaking, it is desirable that the agent be highly reactive, i.e., the H2S (or organic sulfides) should react readily with the agent as the H2S passes through the treatment liquid. At the same time, it is desirable that the agent remain effective even in the presence of a large excess of CO2, that is, it should be selective. The scavenging agent should be easy to handle, that is, be of appropriate viscosity, have a suitable pour point, and low toxicity. Further, it is also desirable that the agent remain in the liquid state and not foam excessively in use to prevent contamination of the treated gas.
Other problems also attend the use of sulfide scavengers. For example, sulfide scavengers can yield reaction products which precipitate and/or polymerize, thereby damaging the permeability of the formation. Additionally, sulfide scavenger reaction products can themselves be corrosive and/or toxic.
When using conventional triazine scavenging treatments, while effective, care must be taken that a sufficient amount of the scavenger is injected into a well. If there is an insufficient amount of triazine injected, i.e., an excess of hydrogen sulfide is present, it has been observed that the dithiazine reaction product from triazine and hydrogen sulfide may react with the excess hydrogen sulfide to form an insoluble product that may damage the formation.
Accordingly, there exists a continuing need for improved compositions and methods for treating sulfide-containing wells.